Carbon Dioxide Fixation by Cells of Streptococcus faecalis var. liquefaciens1

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Fixation of NaH14CO3 by a heavy cell suspension of Streptococcus faecalis var. liquefaciens was studied. Several nutrients, pyridoxal, riboflavine, adenine, uracil, and O2 stimulated 14CO2 incorporation into cells only under conditions that were adequate for synthesis of cell macromolecules. Biotin increased CO2 incorporation in the absence of extensive synthesis of macromolecules, whereas O2 inhibited incorporation under these conditions. When 14CO2 fixation was occurring during synthesis of macromolecules, 71% of the 14C was incorporated into cells and 29% occurred extracellularly. Ninety-three per cent of the cellular 14C was in protein and 5.5% was in nucleic acid. Aspartic acid was the only amino acid in the protein fraction that was radioactive. Eighty-three per cent of the extracellular 14C was resistant to precipitation by trichloroacetic acid. When 14CO2 fixation was occurring in cells that were not carrying on extensive synthesis of macromolecules, 38% of the 14C was incorporated into cells and 59% occurred in the supernatant fluid. Sixty-nine per cent of the cellular 14C was in protein, 21% was in low-molecular-weight compounds, and 9% was in nucleic acid. Addition of unlabeled aspartate to the medium inhibited incorporation of 14CO2. Based on studies of the rate of 14CO2 fixation, the cells fix CO2 into a pool of intermediates which are either used for synthesis, primarily protein, or are excreted into the medium.

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